StringRunner

StringRunner.cs

public static string StringRunner(string s) {
    var output = new StringBuilder();
    for(int i = 0; i < s.Length; i++) {
        char prevCh = i > 0 ? s[i - 1] : '\0';
        char thisCh = s[i];
        char nextCh = i < s.Length - 1 ? s[i + 1] : '\0';
        switch(thisCh) {
            // Do the replacements
            default:
                output.Append(thisCh);
                break;
        }
    }
    return output.ToString();
}